1. Field of the Invention
The present invention relates to a superconducting thin film and a method for preparing the same. More particularly, it relates to a superconducting thin film of compound oxide having a high critical temperature and also possessing lasting stability for a long period and a method for preparing the same.
2. Description of the Related Art
The superconductivity is a phenomenon which is explained to be a phenomenon of a kind of phase change of electrons under which the electrical resistance become zero and the perfect diamagnetism is observed. Thus, under the superconducting condition, electric current of a very high current density can be delivered without any loss of power.
Therefore, if the superconducting power cable is realized, the power loss of the order of 7% which is inevitable in the conventional power cables can be reduced greatly. Realization of superconducting coils for generating a very high magnetic field is expected to accelerate development in the field of fusion power generation in which the electric power is consumed beyond its output under the present technology, as well as in the field of MHD power generation or motor-generators. The development of superconductivity are demanded also in the other industrial fields such as in the field or electric power reservation; in the field of transportation for example magnetic levitation trains, or magnetically propelling ships; in the medical field such as high-energy beam radiation unit; or in the field of science such as NMR or high-energy physics.
In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a device using the Josephson effect in which quantum efficiency is observed macroscopically when an electric current is passed through a weak junction arranged between two superconducting bodies. Tunnel junction type Josephson device which is a typical application of the Josephson effect is expected to be a high-speed and low-power consuming switching device owing to smaller energy gap of the superconducting material. It is also expected to utilize the Josephson device as a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like since variation of electromagnetic wave or magnetic field is reflected in variation of Josephson effect and can be observed as a quantum phenomenon precisely. Development of the superconducting devices is also demanded in the field of high-speed computers in which the power consumption per unit area is reaching to the upper limit of the cooling capacity with increment of the integration density in order to reduce energy consumption.
However, the critical temperature could not exceed 23.2 K of Nb.sub.3 Ge which was the highest Tc for all studies for the past ten years.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189].
It had been known that certain ceramics material of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba--Pb--Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba--Bi type compound oxides also show superconductivity. These superconductors, however, possess rather lower transition temperatures of about 10 K and hence usage of liquidized helium (boiling point of 4.2 K) as cryogen is indispensable to realize superconductivity. The new type compound oxide superconductor discovered by Bednorz and Mueller is represented by [La, Sr].sub.2 CuO.sub.4 which is called as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxides. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30 K which are extremely higher than known superconducting materials.
It was also reported in the newspaper that C. W. Chu et al discovered in the United States of America another superconducting material so called YBCO type represented by YBa.sub.2 Cu.sub.3 O.sub.7-x having the critical temperature of in the order of 90 K in February 1987. Still another type new superconducting material is a compound oxide of Bi--Sr--Ca--Cu--O system and Tl--Ba--Ca--Cu--O system which exhibit such high Tc as more than 100 K and which are chemically much stable than the abovementioned YBCO type compound oxide or the like.
And hence, possibility of existence of high-temperature superconductors have burst on the scene.
A thin film of superconducting compound oxide is prepared by sputtering technique. For example, in Japanese patent laid-open No. 56-109824 which discloses a method for preparing a thin film of superconductor represented by BaPb.sub.1-x Bi.sub.x O.sub.3 (in which 0.05.ltoreq.x.ltoreq.0.35), high-frequency sputtering is carried out in an oxygen containing atmosphere and then the resulting film is further heated at 500.degree. to 550.degree. C. We also described sputtering conditions for preparing superconducting thin films of compound oxide by the sputtering technique in a commonly assigned co-pending U.S. patent application Ser. No. 152,714 filed on May 2, 1988, now abandoned.
The abovementioned conventional sputtering technique itself is satisfactory but is helpless against the change in properties during storage.
In fact, the superconductivity or superconducting property of the abovementioned new type superconducting materials of compound oxide is influenced by the oxygen contents in the crystalline structure since the oxygen in the crystal is not stable and is apt to escape out of the crystal, resulting in that the superconductivity is lost or deteriorated with the passing of time.
Therefore, an object of the present invention is to overcome the abovementioned problems of the conventional technique and to provide a superconducting thin film improved in stability during storage or use and a method for preparing the same.